In a CDMA system employing different lengths of spreading codes, receiver imperfections, such as phase noise, may degrade transmissions more when using a longer spreading code than using a shorter spreading code if the nature of the imperfections is time-varying, such as phase noise on the scale of the spreading code length. For example, in a universal mobile telecommunication system (UMTS) frequency division duplex (FDD) system, the spreading codes may vary from 4 to 512 chips.
In a third generation (3G) high speed downlink packet access (HSDPA) system, adaptive coding and modulation (AMC) is based on a channel quality indication (CQI) estimated by a wireless transmit/receive unit (WTRU). The CQI is expected to reflect the channel quality of the high speed physical downlink shared channel (HS-PDSCH), which uses a spreading factor (SF) of 16. However, the CQI is generated based on a signal-to-interference ratio (SIR) measured on a common pilot channel (CPICH), which has an SF of 256. In an ideal radio environment, this does not present a problem because different processing gains due to different SFs are easily factored into the CQI generation. However, phase noise can impact the SIR measurements made on signals of different SFs by different amounts. Therefore, the CQI measurement based on the CPICH may not reflect the channel quality seen by the HS-PDSCH.